Your gas tank should last a while. I'd suggest a solenoid and a timer. I use a smart plug with a scheduled on/off time. I set my system up on as low as possible and monitor it with a drop checker. Note a drop checker pretty much tells you what your ph level was an hour or two ago. I'll adjust up in minute increments daily until I see a color change (from blue to green) on the drop checker. Note some cheaper needle valves will "creep" on you and won't be super easy to fine tune. Just keep an eye on it for the first few weeks and err on the low side.